fix: Make up/previous navigation consistent with down/next.

[gonfunko]

The basics

• I validated my changes

The details

Resolves

Fixes RaspberryPiFoundation/blockly-keyboard-experimentation#762

Proposed Changes

This PR modifies the locations that are visited when navigating using the up arrow key to match those that are visited by the down arrow key, but in reverse. Specifically, when navigating backwards, we now:

• Visit the last (empty) next connection in a block stack within a statement input
• Do not visit the first (empty or filled) value input in a block with statement inputs, because this results in redundant screenreader output with the block itself. These inputs/blocks are also skipped when navigating with the down arrow for the same reason
• Do not focus the rightmost nested block in a statement block; instead, focus on the root statement block itself.

[maribethb]

I think a similar set of tests is failing on my PR against this branch. Not sure what's up with them, although at least this PR is plausible that some of the tests need updating.

[gonfunko]

Fixed the tests in RaspberryPiFoundation/blockly-keyboard-experimentation#766, which should fully pass once this is merged.